1. Field of the Invention
The present invention relates to a ceramic heater and a glow plug which employs the same. More particularly, the present invention relates to a ceramic heater to be used for igniting a kerosene stove with air circulation fan, and to a glow plug which employs the ceramic heater and is used for assisting the startup of a diesel engine or the like.
2. Description of the Related Art
There has been a trend in recent years of shifting the combustion method of diesel engine from a system provided with an auxiliary combustion chamber to direct fuel injection. There is also a trend to employ multiple valves. The glow plug used in a diesel engine of direct fuel injection system is disposed to penetrate the wall of a cylinder head and face a main combustion chamber. Wall thickness of the cylinder head cannot be made too small, as the cylinder head must have a certain level of strength.
For the reasons described above, the diesel engine of direct fuel injection system has very narrow and long hole through which a glow plug is inserted. In other words, it is important that the glow plug used in the diesel engine of direct fuel injection system be longer and thinner than the one of the conventional type which preheats the auxiliary combustion chamber.
In order to meet the requirement for a longer glow plug and reduce the length of the ceramic heater so as to cut down on the cost, a glow plug having such a structure has been proposed as the ceramic heater is secured at one end of an outer tube made of metal so that a heat generating portion of the ceramic heater protrudes to the outside.
For example, Japanese Unexamined Patent Publication (Kokai) No. 2002-122326 (p8, FIG. 1) describes a glow plug having an outer tube made of metal joined at the distal end thereof, wherein a ceramic heater is secured by means of glass on an opening at the distal end of the outer tube made of metal. The ceramic heater has a heat generating resistive member, such as coil made of a metal having high melting point (for example, tungsten) or an electrically conductive ceramics, embedded at one end of a cylindrical ceramic member made of an electrically insulating ceramics. The heat generating resistive member has a positive lead wire and a negative lead wire connected thereto. A round protrusion is formed at an end of the ceramic member on the side opposite to that where the heat generating resistive member is embedded, and the distal end of the positive lead wire is exposed on the side face of the protrusion. The negative lead wire is exposed on the side face of the ceramic member.
Connected to the distal end of a positive electrode lead-out fixture of the glow plug is a terminal formed in a cup shape (bottomed tube shape). The cup-shaped terminal of the positive electrode lead-out fixture is fitted into the protrusion formed at the end face of the ceramic heater and joined together by brazing. This establishes electrical connection between the positive electrode lead-out fixture of the glow plug and the positive lead wire of the ceramic heater. The negative lead wire exposed on the side face of the ceramic member is connected to the outer tube made of metal of the glow plug.
The ceramic heater described above can be manufactured as follows. The ceramic heater is sintered by firing with the positive lead wire disposed at a position offset from the center. After sintering, the ceramic heater is ground or otherwise machined on the end face so as to form a protrusion, such that the distal end of the positive lead wire is exposed on the side face of the round protrusion.
Japanese Unexamined Patent Publication (Kokai) No. 2001-324141 describes a glow plug having a positive lead wire of a ceramic heater and a positive electrode lead-out fixture connected with each other via a connection hole. Specifically, the ceramic member has the connection hole formed at the rear end thereof, and the positive electrode lead-out fixture is inserted into the connection hole and is connected to the positive lead electrode. The connection hole (positive electrode lead-out hole) is formed by sintering while the hole is filled with a metal having high melting point such as Mo, and dissolving the metal such as Mo by means of an acid.